The overall long-term goal of this multidisciplinary research program is to apply new optical technology to investigate early changes associated with cellular immune response and to monitor response to therapy over time in vivo and at the cellular level. Specifically, we proposed to develop in vivo immunofluorescence microscopy for imaging specific cell surface markers expressed by vascular and lymphatic endothelial cells, circulating leukocytes, and tissue dendritic cells. The technology that enables noninvasive, real-time cellular imaging is a video rate confocal and multiphoton fluorescence microscope that we recently developed in our laboratory. We will apply this technology to study important steps involved in the regulation of cellular immune response in vivo, including endothelial cell activation, leukocyteendothelial interaction, and dendritic cell recruitment and migration. In addition, we proposed to develop an in vivo flow cytometer, a novel technology for real-time, noninvasive detection and quantification of circulating cells that are tagged with fluorescent antibodies or antibody fragments. We will use this system to monitor changes in specific T cell population in response to immunomodulation, to detect circulating tumor cells and to investigate the correlation between tumor cell shedding and metastatic potential.